Conventional CdTe/CdS solar cells typically comprise a transparent glass substrate carrying a transparent conductive oxide (TCO) film, a CdS film serving as the n-conductor, a CdTe film serving as the p-conductor and a metallic back-contact. A solar cell of this general description is disclosed, for example, in U.S. Pat. No. 5,304,499, which issued on Apr. 19, 1994.
While this “float” glass concept may be adapted for commercial use as a transparent substrate, frequent diffusion of Na into the TCO film has often resulted. Consequently, despite its relatively low cost, special glasses are often preferred over such a “float” glass arrangement.
Perhaps the most common TCO material is In2O3 which contains about 10% Sn (ITO). This material is usually characterized by a very low resistivity on the order of 3×10−4 Ωcm and a relatively high transparency (>85%) in the visible spectrum. While useful this material is made by sputtering and, after several runs, the ITO target forms noodles which contain excess In. In addition, a discharge may occur between noodles during sputtering which can damage the film.
Another material that is commonly used for the transparent conductive oxide film is fluorine doped SnO2. Although helpful, this material exhibits a higher resistivity close to about 10−3 Ωcm. As a result, a 1 μm thick layer is necessary to keep the sheet resistance at about 10 Ω/square. Generally, a high TCO thickness decreases the transparency and, in turn, the photocurrent of the solar cell. In addition, a novel material, namely Cd2SnO4, developed by the NREL group (X. Wu et al., Thin Solid Films, 286 (1996) 274-276)) has been utilized. However, since the target is made up of a mixture of CdO and SnO2, CdO being considered highly hygroscopic, the stability of the target has often been found unsatisfactory.
Generally speaking, the CdS film is deposited either by sputtering or Close-Spaced Sublimation (CSS) from a CdS granulate material. The latter technique allows thin films to be prepared at a substrate temperature considerably higher than that used in simple vacuum evaporation or sputtering. This is because the substrate and evaporation source are positioned very close to one another, i.e., at a distance of 2-6 mm, and deposition is performed in the presence of an inert gas such as Ar, He or N2 at a pressure of about 10−1-100 mbar. A higher substrate temperature usually allows growth of a better quality crystalline material. A significant characteristic of close-spaced sublimation is a very high growth rate up to about 10 μm/min, which is suitable for large-scale production.
Next, a CdTe film is deposited on the CdS film through close-spaced sublimation at a substrate temperature of 480-520° C. CdTe granulate is generally used as a source of CdTe which is vaporized from an open crucible.
An important step in the preparation of high efficiency CdTe/CdS solar cells is the treatment of CdTe film with CdCl2. Traditionally, most research groups would perform this step by depositing a layer of CdCl2 on top of CdTe by simple evaporation or by dipping the CdTe in a methanol solution containing CdCl2, and then annealing the material in air at about 400° C. for between about 15 and about 20 min. It is generally believed that CdCl2 treatment improves the crystalline quality of CdTe by increasing the size of the small grains and removing defects in the material.
After CdCl2 treatment, the CdTe is etched in a solution of Br-methanol or in a mixture of nitric and phosphoric acid. Etching is necessary as CdO or CdTeO3 are generally formed on the CdTe surface. CdO and/or CdTeO3 must be removed in order to provide for good back-contact onto the CdTe film. Also, it is believed that, since etching produces a Te-rich surface, formation of an ohmic contact when a metal is deposited on CdTe is facilitated.
The electric back-contact on the CdTe film is generally obtained by deposition of a film of a highly p-dopant metal for CdTe such as copper, e.g., in graphite contacts, which, upon annealing, can diffuse in the CdTe film. Use of a Sb2Te3 film as a back-contact in a CdTe/CdS solar cell is set forth by applicants in N. Romeo et al., Solar Energy Materials & Solar Cells, 58 (1999), 209-218).
Industrial interest in thin films solar cells has increased in recent years, especially in view of the relatively high conversion efficiency achieved. Recently, for instance, a record 16.5% conversion efficiency was reported (see X. Wu et al., 17th European Photovoltaic Solar Energy Conversion Conference, Munich, Germany, 22-26 Oct. 2001, II, 995-1000). Accordingly, a number of attempts have been made to provide processes suitable for large-scale, in-line production of CdTe/CdS thin film solar cells. A state-of-the art report of these efforts may be found in D. Bonnet, Thin Solid Films 361-362 (2000), 547-552. While helpful, they include crucial steps that also affect either the stability and efficiency of CdTe/CdS thin film solar cells or their costs, thereby hindering achievement of a commercially viable process.
A significant problem of these processes is the etching step to which the CdTe surface must be submitted for removing CdO or CdTeO3 oxides that form thereon. Although etching requires the steps of immersing substrates carrying the treated CdTe/CdS films into acid solutions, rinsing and drying, machinery suitable for such continuous operation does not currently exist. Another difficulty that can negatively affect the stability of TCO films, as well as the cost of the final product, are the aforementioned disadvantages encountered using known TCOs. Moreover, known TCOs typically require the use of special glasses, such as borosilicate glass, to avoid Na diffusion and associated damage to the film that often occurs when soda-lime glass is used.
A further drawback of conventional processes relates to the source from which the CdS film and the CdTe film are produced through close-spaced sublimation. When relatively small pieces of these materials which contain dust, are used as a sublimation source, because of a different thermal contact, some micro-particles can overheat and, together with the vapor, split onto the substrate. In an attempt to avoid this inconvenience, complicated metallic masks are often used which make continuous operation problematic.